First – What is “Exposure”
Something I find useful to consider is that “Exposure” is the amount of light that you put onto the sensor or film stock in your camera. It isn’t brightness, it is how much light. If you think about it, if you use a light meter to find you exposure settings, the light meter has no idea how bright the pictures will be, all it does is give you the shutter and aperture values needed to put the correct amount of light onto the sensor or film stock.
How Cine EI Works.
Next we need to think about how Cine EI works. You have to remember that when shooting using Cine EI the only thing that changes when you change the EI value is the brightness of the LUT and it is also worth considering that different LUTs may be completely different brightness. There is no change to the sensitivity of the sensor and no direct change to the brightness of what is recorded. To change the brightness of what is recorded YOU must change the aperture, shutter speed or ND etc. Normally you would monitor your images via a LUT and then you must adjust the exposure so the image on the viewfinder looks correct at the new Exposure Index, or use the waveform to measure the LUT and use this to set the exposure for the new EI. And by changing the exposure you are adding an exposure offset putting more (or less) light on to the sensor than would be normal at the base EI.
AE In Cine EI.
If you wish to use auto exposure in the Cine EI mode then you need to understand that the camera’s auto exposure system measures what is being recorded. It does not measure the LUT levels. The auto exposure system is unaware of your desire to expose the sensor more or less brightly than normal and will always base the exposure on the base ISO, not the Exposure Index. As a result if you are using AE and you go from 800 EI to 400 EI the image seen via the internal LUT will get darker by one stop, the AE will NOT compensate for the lower EI. If you were to manually brighten the exposure by one stop the cameras exposure meter will think you are now over exposed – because you are!
The only way around this is to add an offset to the AE system to account for the offset added by the different Exposure Index. For example if you want to shoot at 400EI (The LUT becoming 1 stop DARKER) then you would need to add a +1.0 stop offset to the cameras AE settings to offset the exposure 1 stop brighter. Each time you halve the EI you should add an extra +1 stop of offset. Each time you double the EI you should include an extra -1 stop offset.
There are a couple of ways to do this but the quickest is to use the Quick Menu function that is by default assigned to button 5 on the hand grip or button 8 on the handle. Press the direct menu button and then use the thumbstick to go the AE+0 indication just above the shutter speed indicator and add your offset.
Or you can long press the menu button to go into the cameras main menu then go to the – Shooting – Auto Exposure page and add your offset to the Level setting.
I don’t recommend the use of Auto Exposure in Cine EI. For a start AE uses the average brightness of the scene to set the exposure level, often this isn’t appropriate for Log. When shooting with log generally you want to ensure that it is your mid range is exposed at the right level and you don’t want bright highlights to result in an under exposed mid range. Additionally if the exposure changes mid shot this can make grading very difficult. If you do use auto exposure in Cine EI, then as well as adding any necessary offsets I also recommend slowing down the responsiveness of the AE using the “Speed” setting in the Auto Exposure menu. Using a value such as -60 will slow down the rate at which the AE will change the exposure which helps avoid rapid auto exposure changes for momentary light changes within the scene.
It is really important to remember that Exposure is NOT brightness. Exposure is how much light you put on the sensor. A light meter doesn’t know how bright you want your pictures to be. All it knows is the correct amount of light to put on to the sensor for the “correct” exposure. If using an external light meter provided you put the right values into your light meter it will give you the correct exposure settings, even though it has no idea how bright your pictures will be and the camera’s internal exposure meter acts in a similar way, so offsets are needed to match each EI you use.